Wednesday, March 16, 2011

A cup of coffee, please

Warning: high levels of nostalgia

There is a quite well-known urban myth regarding the ritual of drinking coffee. Some say that it's better to add milk right away if you want to cool the drink faster, others say it's better to wait a bit.
Almost two years ago I decided to clear all doubt concerning this argument. It was August 2009 when I got my first pair of the (well known for regular readers) digital thermometers, and this experiment was perfect to make them useful.

So I gathered all the required instruments: two identical digital thermometers, two identical ceramic cups (0.15 L each), two identical stainless steel spoons and a digital camera.

I cooked a large pot of coffee and poured equal amounts (0.1 L) of the boiling liquid into each cup wherein I have already put the probes of the digital thermometers. Then I immediately poured two spoons of refridgerated milk (kept at 8 °C) in the first cup and after approximately five minutes I did the same to the second cup. After every addidion of milk I stirred the mixture with the spoon for two full turns (clockwise).
An artist's impression; the original image is probably lost forever but the reproduction is pretty accurate.
Temperature of the room was 23.5 °C. The temperature changes were recorded in real time with a digital camera and the data was then processed for easier understanding. The thermometers were not calibrated, so there was a small difference between the initial temperatures. Also, the coffee was poured in the second cup with a small delay. I adjusted the data of the second cup accordingly.
The results:
The plot of the temperatures.
As you can see the temperature differences are minimal. The initial temperature drop after the addition of milk was about 5 °C for both cups. On average the coffee with the milk added right away was cooler for 2 °C before the addition of milk to the second cup and about 3 °C hotter immediately after. Because the thermodynamic phenomenon that a hotter body cools more quickly and the poor thermal conductivity of ceramic the first cup was actually cooler after seven minutes from the addition of the milk in the second cup. Unfortunately I was still a rookie scientist at the time and I did not have a control cup.
In conclusion it's pretty irrelevant when will you put the milk in because you probably won't even notice the difference. But it was still a fun experiment and all doubts about the myth are cleared.
Some possible extensions of the experiment: measuring the temperatures in plastic cups; measuring the temperatures when adding ice cubes into the drink immediately and after 5 minutes; measuring the temperatures with different mixing techniques; and so on.